What to do if your girlfriend doesn't trust you simply because you talked to other girls.?

So I talked to a few girls at school, one, because she sat next to me in class, we became friends, but only in that class room, and she wrote on my binder, i told my girlfriend about it, and i would tell her every little thing i would say to the friend and she ultimately made me stop being her friend. she freaked out when she saw that the girl wrote on my binder. she said i was "flirting" with them, and always accuses me of it, when i know for a fact, i wasnt flirting. and now she has trust problems. another story was this girl in my body conditioning class, we were outside and the girl and i started throwing dirt clogs at each other. i gave her a welt on the stomach, and she showed me. and i went and told my girlfriend everything that happened so that she knew. again she accused me of flirting. she even text the girl to never talk to me again. on the other hand, i trust her very much, and told her that she could like have whatever guy friends she wanted, and so she became friends with my friends, and one night she was i.m ing them on facebook, she said, "it was so nice to see you again." and she told him. "your one of my best friends." after the guy told her she was one of his best friend. we have argued about her jealousy many times, and she said that she will change, but she doesn't, just today we argued about a girl that added me on Facebook and i didn't even know who she was. she asked me, who is..., and i said i don't know. and she just dragged it on and on. i don't know what to do anymore. i love her a lot, we have been together for more then a year, but sometimes i cant take it anymore.